Implement if else support

src/interpreter.rs

@@ -3,13 +3,22 @@ use crate::parser::Datum::*;
 use crate::primitives::*;
 use std::mem;
 
-fn lookup_variable_value(exp: Datum) -> Datum {
+fn is_true(exp: &Datum) -> bool {
+    match exp {
+        Boolean(b) => *b,
+        Number(_) => true,
+        List(_) => true,
+        _ => panic!("IS-TRUE -- unknown-truth-value {:?}", exp),
+    }
+}
+
+fn lookup_variable_value(exp: &Datum) -> Datum {
     if let Symbol(string) = exp {
         match string.as_str() {
             "+" => Procedure(add),
             "*" => Procedure(mul),
             "-" => Procedure(sub),
-            _ => panic!("unbound-variable {:?}", string),
+            _ => panic!("LOOKUP-VARIABLE-VALUE -- unbound-variable {:?}", string),
         }
     } else {
         panic!("LOOKUP-VARIABLE-VALUE -- not-a-symbol {:?}", exp)
@@ -19,7 +28,7 @@ fn lookup_variable_value(exp: Datum) -> Datum {
 fn application(exp: Datum) -> Datum {
     if let List(vector) = exp {
         if vector.len() == 0 {
-            panic!("Application needs at least a procedure")
+            panic!("APPLICATION -- no-procedure")
         }
 
         let mut args: Vec<Datum> = vector.into_iter().map(interpret).collect();
@@ -27,18 +36,56 @@ fn application(exp: Datum) -> Datum {
             // FIXME: call procedures with args only
             proc(args)
         } else {
-            panic!("The object {:?} is not applicable", args[0])
+            panic!("APPLICATION -- not-aplicable {:?}", args[0])
         }
     } else {
-        panic!("Cannot apply {:?}", exp)
+        panic!("APPLICATION -- cannot-apply {:?}", exp)
+    }
+}
+
+fn has_tag(exp: &Datum, tag: &str) -> bool {
+    if let List(l) = exp {
+        if l.len() == 0 {
+            return false;
+        } 
+        if let Symbol(s) = &l[0] {
+            if s == tag {
+                return true;
+            }
+        }
+    }
+    false
+}
+
+fn interpret_if(mut args: Vec<Datum>) -> Datum {
+    let predicate = mem::take(&mut args[1]);
+    let alternative = mem::take(&mut args[2]);
+
+    if is_true(&predicate) {
+        interpret(alternative)
+    } else {
+        if args.len() == 4 {
+            let consequent = mem::take(&mut args[3]);
+            interpret(consequent)
+        } else {
+            Unspecified
+        }
     }
 }
 
 pub fn interpret(exp: Datum) -> Datum {
     match exp {
-        Boolean(_) | Number(_) => exp,
-        Symbol(_) => lookup_variable_value(exp),
+        Boolean(_) => exp,
+        Number(_) => exp,
+        Symbol(_) => lookup_variable_value(&exp),
+        List(v) if has_tag(&exp, "if") => interpret_if(v),
+        List(_) if has_tag(&exp, "set!") => panic!("assignment-not-supported"),
+        List(_) if has_tag(&exp, "define") => panic!("define-not-supported"),
+        List(_) if has_tag(&exp, "cond") => panic!("cond-not-supported"),
+        List(_) if has_tag(&exp, "let") => panic!("let-not-supported"),
+        List(_) if has_tag(&exp, "begin") => panic!("begin-not-supported"),
         List(_) => application(exp),
+        // TODO: quoted
         _ => panic!("unknown-expression {:?}", exp),
     }
 }